An ignition plug is used to ignite an air-fuel mixture in a combustion chamber of, for example, an internal combustion engine. The ignition plug includes, for example, a tubular metallic shell, and an insulator disposed around the inner circumference of the metallic shell.
A reduction in the diameter of an ignition plug is desired for improving the degree of freedom for design of an internal combustion engine. A reduction in ignition plug diameter, however, may cause problems during production of a metallic shell; for example, insufficient strength of the metallic shell. Chips generated during production of the metallic shell may deposit on a tool, resulting in low productivity.
The present specification discloses a technique for preventing a reduction in the strength of a metallic shell and a reduction in productivity.